Grafted polyolefins are especially useful as blend components because they can impart adhesion to the blend. For example, U.S. Pat. Nos. 4,774,144, 4,842,947, 5,336,721, and 5,439,974 describe several blends employing graft polyolefins. Unreacted grafting monomers and oligomers often need to be removed from the grafted polyolefin products because these impurities can have deleterious effects on color, adhesion, and film clarity.
Methods for removing oligomers from grafted polyolefins are known. For instance, Japanese Pat. No. 2185505 teaches immersing molten grafted polyolefin in a carbonyl containing solvent such as methyl ethyl ketone to remove the unreacted monomers and oligomers. Japanese Pat. No. 4202202 teaches extracting oligomers from grafted polyolefin by refluxing, through the polymer, a mixture of toluene and methyl ethyl ketone, followed by rinsing with acetone.
Copending application Ser. No. 10/846,735, filed May 14, 2004, teaches extracting the unreacted monomers and oligomers from the grafted polyolefin with azeotropic solvent mixtures.
It is of great advantage to be able to economically recover these solvents. One challenge in recovering the solvents is that at the end of distillation the oligomers are concentrated in the bottoms and can cause fouling. To overcome this problem, a relatively large amount of solvent often needs to remain with the oligomers; the mixture is often treated as a waste stream. Incomplete recovery of the solvents not only leads to poor economics but also to a waste disposal problem.
Hence, a new method is needed for more efficiently recovering the solvents. Ideally, the method would be performed without increasing the operational cost.